GOLDEN, COLO.
— WILL sunlight be a central source of United States energy for the 21st century? Here at the sprawling, low-rise campus of the Solar Energy Research Institute (SERI), the answer is a resounding ``Yes.''

Politically, the answer may be coming just in time. Conventional sources of power generation - coal, oil, and natural gas - are coming under increasing fire from an environmentally conscious public concerned about global warming, acid rain, and air pollution. Also of concern: The national security implications of dependence on oil, nearly 40 percent of which is imported.

But nuclear energy, once touted as an environmentally clean alternative, is suffering from continuing revelations following the explosion of a Soviet nuclear reactor at the Chernobyl power station in 1986. The political difficulties surrounding the start up of the Seabrook Nuclear Power Plant in New Hampshire are seen by many as a portent of greater challenges facing nuclear-energy installations in the future.

``The progress in the last decade has been truly remarkable,'' says Robert Stokes, deputy director of research for SERI, a 13-year-old research facility owned by the US Department of Energy. SERI is currently budgeted at $100 million a year. ``We believe that the government is now set up on a course that will gradually increase the research and development investment in renewable energy to the point that many of these technologies will become cost-effective,'' Mr. Stokes says.

Researchers at SERI, taking an expanded view of the word ``solar,'' use it to cover any of the alternative energy resources that ultimately derive their energy from the sun - including wind power, combustion of organic (biomass) products, wave- and tidal-power, thermal gradients in the ocean, and a number of other technologies.

Already, say researchers, some of these technologies are cost-effective for certain applications. ``We're very much closer today than we were eight or 10 years ago to eventual economic parity with a variety of other energy technologies,'' says Tom Bath, manager of SERI's Analysis and Evaluation Office. Among the cost-effective technologies:

Photovoltaics (PV), the direct conversion of sunlight into electricity using solid-state ``photo-cells,'' has been powering satellites since Vanguard I was launched in 1958. It is already the technology of choice for small applications far from conventional power grids - rural water pumps, communication relay stations, vaccine refrigerators in third-world nations - and is even competitive for some remote homes in the United States. ``If you are more than a third of a mile from the grid,'' Dr. Bath says, ``it pays you to buy PV and batteries today rather than try to hook up.''

Solar thermal power plants like the Luz International plant at Kramer Junction, Calif., use acres of trough-shaped mirrors to focus the sun's rays on vacuum-insulated tubes of oil. Heated to 735 degrees F, the oil is used to generate superheated steam that drives a turbine generator. At a cost of less than 8 cents per kilowatt hour, say plant officials, their power is already cheaper than nuclear power and is becoming competitive with oil and coal. Now producing 274 megawatts of power in southern California, Luz will reach almost 680 megawatts by 1994 - enough to meet the entire residential needs of a city the size of San Francisco or Phoenix.

Wind power, typically generated at windmill farms located in high mountain passes in the West, is already a viable industry. In California alone, nearly 16,000 wind turbines generate nearly 2 billion kilowatt hours each year - as much energy as a medium-sized nuclear plant. Because they are nonpolluting, says Paul Gipe of the American Wind Energy Association in Tehachapi, Calif., these turbines alone offset 1.8 billion pounds of greenhouse gases that would otherwise pour into the atmosphere from conventional power plants.

Biomass conversion takes many forms, some of which are already cost-effective. Wood-burning and municipal solid-waste conversion, already supplying 3 to 5 percent of the nation's energy needs, have a potential of about 12 percent. More complex technologies - including the biological conversion of energy crops into alcohol fuels and biogas, the cultivation of microalgae and oilseed crops to produce diesel fuel, and the thermal conversion of biomass into synthetic gas - are already in limited operation. Taken together, they could ultimately produce as much as 20 percent of the nation's energy needs.

Hydrogen, a gas that produces only water vapor when it burns and produces no greenhouse gases or air pollution, is increasingly being considered as a transportation fuel and as a means to store energy. Easily made by an electrolysis process that uses direct current to split water molecules into hydrogen and oxygen atoms, it needs only a cheap source of electricity to be economically attractive. Hydrogen-powered automobile prototypes are already in existence. A recent report from the World Resources Institute in Washington notes that by the turn of the century, as the cost of producing photovoltaic electricity falls to between 2 and 3.5 cents per kilowatt hour in the sunnier regions of the nation, hydrogen could become a cost-competitive fuel.

The ``bottom line'' for all these alternative sources, Bath says, depends on whether the nation's research and development (R&amp;D) investment remains constant or increases. ``With a business-as-usual kind of R&amp;D by the United States,'' he says, ``renewable goes from about 8 percent of the US energy mix to around 13 by 2030. If you intensify R&amp;D, we can get up to 28 percent of the energy mix by 2030.'' The 28 percent figure, he says, is ``roughly equivalent to what coal or gas-and-nuclear do today.''

Researchers at SERI acknowledge that the 1980s were a quiet period in the history of solar energy. The flurry of public interest following the 1973-74 Arab oil embargo led to an upsurge in research into alternative energy sources. As oil prices fell, however, the urgency to develop solar resources also declined - leaving alternative energy with the reputation of a kind of idealistic crusade.

Over the last decade, however, steady progress has been made on a number of fronts. In photovoltaics, for example, solar cells are now available that are more than 27 percent efficient in converting the sun's energy. By contrast, nature's solar conversion process, known as photosynthesis, is ``on the order of 1/2 of 1 percent,'' says Dr. Stokes.

In fact, it is that very maturing of the technology that drives the success of solar programs. ``In contrast to conventional energy systems, these technologies are generally not resource-limited - they're technology-limited,'' Bath says. He adds, however, that ``the resource in many cases is diffuse, and it's erratic. Those two factors provide the source of the technological challenge.''

For these reasons, some technologies work well only in certain regions. Diffuse sunlight, of the sort usually available under cloud cover in the nation's Northeast, is an adequate source of PV energy, but it will not power today's solar thermal plants, which need to be located in the Southwest.

And wind power, which is erratic, will benefit greatly from continuing research into energy storage systems such as hydrogen conversion, new batteries, and superconducting rings.

Watching the rate of progress in solar technologies, however, Mr. Zweibel is optimistic. ``Solar,'' he says flatly, ``can meet the global-warming crisis.''